1. Field of the Invention
This invention relates to an ink jet ink, an ink set, an ink jet recording method, an ink cartridge, a recording unit and an ink jet recording apparatus.
2. Description of the Related Art
The ink jet recording method is a recording process in which ink droplets are applied to recording mediums such as plain paper and glossy mediums to form images thereon, and is rapidly coming into wide use because of its contribution to low price and achievement of improvement in recording speed. As images thereby obtainable have made progress toward higher image quality and in addition thereto digital cameras have rapidly come into wide use, the ink jet recording method has also become widely popular as a process for reproducing images that is comparable to silver salt photography.
In recent years, in such an ink jet recording method, it is required on the one hand to achieve much higher image quality than ever on the images thereby obtainable, and on the other hand it is further required to simultaneously achieve improvement in recording speed. As a method by which ink jet recorded images can be made higher in image quality, it is proposed that first nozzles and second nozzles both having ink channels different in length from each other are arranged in a staggered manner so as to achieve both good refill performance and high-density nozzle arrangement (see Japanese Patent Application Laid-open No. 2006-315395). Herein, the refill performance refers to what may be presented by such a method as given below. For example, it may be presented by the time interval that extends after ink has been ejected and until a meniscus of the ink, which has receded into a nozzle as a result of ejection, returns to a proper position in the vicinity of an ejection orifice to come to enable the next ejection of ink to be properly performed (i.e., refill period).
Meanwhile, in the ink jet recording method, it has come to be highly required to improve image quality of the images to be formed on recording mediums, in particular, on plain paper. Stated specifically, it is sought to improve the uniformity in color of the images to be formed on plain paper.
Further, in order that the ink droplets to be ejected from an ink jet recording head can be made into smaller droplets, the performance required for the ink jet ink has become higher, where the ink is required to have severer properties in respect of ink reliability such as anti-clogging and ejection stability. As an example thereof, the ink is required to be improved in its intermittent-ejection stability. In order to improve both the intermittent-ejection stability and the image quality on plain paper, a proposal is made on an ink jet ink which contains a surfactant having a specific structure and contains urea (see Japanese Patent Application Laid-open No. H08-120204).
About such various problems as above in the ink jet recording method, in order to achieve both the intermittent-ejection stability of ink and the image quality on plain paper while making achievement of high-speed recording and high image quality by making the refill period shorter, the present inventors have made studies on the following. First, they have attempted to perform ink jet recording by using a recording head in which, in addition to the constitution disclosed in Japanese Patent Application Laid-open No. 2006-315395, nozzles having relatively short individual channels and nozzles having relatively long individual channels are alternately arranged at intervals of less than 30 μm, and by using the ink disclosed in Japanese Patent Application Laid-open No. H08-120204. As the result, the present inventors have found that the following new problem comes about when constituted as described above.
That is, as a first problem, it has turned out that a droplet of ink having been ejected comes divided into an ink droplet which is largest in volume (a primary droplet) and a plurality of ink droplets which are relatively small in volume (satellite droplets) and further the satellite droplets impact on a recording medium off the course to cause positional deviation of satellite droplet impact conspicuously. In such a case, the satellite droplets having formed impact on the recording medium in the state they deviate in the direction perpendicular to the direction of primary scanning of the recording head, and hence the images obtained comes greatly low in image quality. Incidentally, such a phenomenon of the positional deviation of satellite droplet impact is not concerned with the speed of movement of the recording head.
Besides the foregoing, the present inventors have also found that the following second problem comes about additionally. That is, where the recording head set up as described above is used, in the nozzles having relatively short individual channels the speed of ejection of the ink ejected differs greatly for each of the ejection made a plurality of times, so that the position of droplet impact of the ink may greatly deviate. Such a problem may come about. In such a case, the ink droplets impact on the recording medium in the state they deviate in the direction parallel to the direction of primary scanning of the recording head. Hence, the more the recording speed is improved (i.e., the higher it is), the more greatly the image quality comes low.
The present inventors have further found that, where the nozzle arrangement of the recording head is made high-density by the method disclosed in Japanese Patent Application Laid-open No. 2006-315395, the area of contact between nozzle materials and a lower-part base member comes relatively small, and this makes the nozzle materials tend to come off, resulting in a great lowering in durability of the recording head.
As discussed above, the studies made by the present inventors have revealed that an attempt to achieve a higher image quality of ink jet recorded images and an improvement in recording speed by changing the construction of the recording head brings about the new problems as stated above. More specifically, the new problems come about such as the positional deviation of satellite droplet impact, the positional deviation of ink droplet impact that is due to changes in ink ejection speed and also the lowering in durability of the recording head. Then, it has come to light that such problems are the cause that makes it more difficult to all well achieve both the higher image quality of ink jet recorded images and the enhancement of recording speed and achieve the uniformity in color on plain paper, as especially strongly required to do so in recent years.